Ceramics International 42 (2016) 14071–14076

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Control of refractive index by annealing to achieve high figure of meritfor TiO2/Ag/TiO2 multilayer films

Jae-Ho Kim a, Dae-Hyun Kimb, Sun-Kyung Kim c, Dukkyu Bae d, Young-Zo Yoo e,Tae-Yeon Seong a,b,n

a Department of Materials Science and Engineering, Korea University, Seoul 02841, South Koreab Department of Nanophotonics, Korea University, Seoul 02841, South Koreac Department of Applied Physics, Kyung Hee University, Gyeonggi-do 17104, South Koread Hexa Solution Co. Ltd., Gyeonggi-do 16229, South Koreae Duksan Hi-Metal Co. Ltd., Yeonam-dong, Buk-gu, Ulsan 44252, South Korea

a r t i c l e i n f o

Article history:Received 30 March 2016Received in revised form19 May 2016Accepted 3 June 2016Available online 7 June 2016

Keywords:TiO2

Refractive indexAgTransparent conducting electrode

x.doi.org/10.1016/j.ceramint.2016.06.01542/& 2016 Elsevier Ltd and Techna Group S.r

esponding author at: Department of Materiniversity, Seoul 02841, South Korea.ail address: tyseong@korea.ac.kr (T.-Y. Seong).

a b s t r a c t

We modified the refractive index (n) of TiO2 by annealing at various temperatures to obtain a high figureof merit (FOM) for TiO2/Ag/TiO2 (45 nm/17 nm/45 nm) multilayer films deposited on glass substrates.Unlike the as-deposited and 300 °C-annealed TiO2 films, the 600 °C-annealed sample was crystallized inthe anatase phase. The as-deposited TiO2/Ag/as-deposited TiO2 multilayer film exhibited a transmittanceof 94.6% at 550 nm, whereas that of the as-deposited TiO2/Ag/600 °C-annealed TiO2 (lower) multilayerfilm was 96.6%. At 550 nm, n increased from 2.293 to 2.336 with increasing temperature. The carrierconcentration, mobility, and sheet resistance varied with increasing annealing temperature. The samplesexhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09 nm. The 600 °C-annealedmultilayer yielded the highest Haacke's FOM of 193.9�10�3 Ω�1.

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

Photonic devices, such as solar cells, organic light-emittingdiodes, and liquid crystal displays require transparent conductingelectrodes (TCEs) [1–3] that should exhibit both low resistance andhigh transmittance. In this regard, indium tin oxide (ITO) is typi-cally used as a TCE in optoelectronic applications because of itslow resistivity (on the order of 10�4Ω cm) and high transmittanceof over 80% in the visible spectrum [4]. However, the limitedsupply of the expensive element In will increase the fabricationcosts of future applications. Thus, to develop an alternative to thecost-ineffective ITO, many different types of transparent con-ducting oxides (TCOs), such as Sn, Zn, Ti, and Nb-based oxides[2,3,5–8], have been investigated. However, the optoelectricalproperties of these TCOs need to be further improved to competewith those of ITO. For the realization of high-performance TCEs,transparent oxides sandwiching a thin metal film, i.e., oxide/metal/oxide (O/M/O) multilayers, have been extensively studied, in-cluding Zn [9,10], Sn [11], Nb [12], Ti–In–Zn [13], Mo [14], W[15,16], Zn–Al [17], Al [18], ZrN [19], and Zn–Mg–Be-based oxides

.l. All rights reserved.

als Science and Engineering,

[20]. A thin Ag layer (r20 nm) is typically used as the middlelayer for O/M/O multilayers because it exhibits low resistance andhigh transmittance in the visible spectrum. The electrical andoptical properties of O/Ag/O multilayers have been shown to bebetter than those of respective TCO-only layers. For example, Yuet al. [11] investigated the dependence of the electrical and opticalproperties of SnO2/Ag/SnO2 multilayer films on the Ag and SnO2

layer thicknesses and reported that the SnO2/Ag/SnO2 (50 nm/5 nm/50 nm) multilayer film exhibited a Haacke's figure of merit(FOM) of 60�10�3Ω�1 with a sheet resistance of 9.67Ω/sq andtransmittance of 94.8% in the visible spectrum. Miao et al. [17] alsoinvestigated the dependence of the electrical and optical proper-ties of Al-doped ZnO (AZO)/Ag/AZO multilayer films on the Ag andAZO layer thicknesses and reported that the AZO/Ag/AZO (30 nm/10 nm/30 nm) samples exhibited the highest transmittance of80.5% in the visible spectrum. Furthermore, TiO2/Ag/TiO2 multi-layers [21–25] are also of great interest because TiO2 exhibits ahigh transmittance in the visible spectrum (490%) and goodchemical stability [26]. The optical and electrical characteristics ofTiO2/Ag/TiO2 multilayers have been widely investigated as afunction of the thicknesses of the TiO2 and Ag films. For instance,Dima et al. [24] investigated the effects of the thicknesses of TiO2

and Ag films on the optical properties of TiO2/Ag/TiO2 multilayersand reported a transmittance of �50% at 550 nm. Jia et al. [25]investigated the effect of the Ag layer thickness on the optical and

J.-H. Kim et al. / Ceramics International 42 (2016) 14071–1407614072

electrical properties of TiO2 (10 nm)/Ag/TiO2 (10 nm) multilayersand reported that the 8-nm-thick Ag multilayer exhibited thelowest sheet resistance (30Ω sq–1) and highest transmittance(�90%) in the 500–700 nm region. In our previous studies [27], weinvestigated the effect of the Ag thickness on the optical andelectrical properties of TiO2/Ag/TiO2 multilayers and reported thatoptimal TiO2/Ag/TiO2 films exhibited high optical transmittancesin the visible spectrum and low sheet resistance (o5Ω/sq),consequently resulting in a Haacke's FOM of 159�10–3Ω�1.

In this study, the refractive index (n) of TiO2 was modified toachieve high transmittance and low resistance of TiO2/Ag/TiO2

films. To modify n, the TiO2 films were annealed at various tem-peratures for 1 h in air. Spectroscopic ellipsometry and X-ray dif-fraction (XRD) were used to analyze the TiO2 films, and Haacke'sFOM was used to characterize the performance of the multilayers.

Fig. 1. TEM image of TiO2/Ag/TiO2 multilayer film.

Fig. 2. XRD patterns of 45-nm-thick TiO2 films annealed at various temperatures.

2. Experimental procedure

The TiO2/Ag/TiO2 multilayer thin films were consecutively de-posited on glass substrates (Coring Eagle XG) using a radio fre-quency (RF) magnetron sputtering system. Ceramic TiO2 targets(99.999% purity) and pure Ag targets (99.99% purity) were used atroom temperature under a base pressure of less than 1�10–6 Torr.Before being loaded into the sputtering chamber, the glass sub-strates (1.5 cm�1.5 cm) were cleaned with acetone, methanol,and deionized water for 5 min per cleaning agent in an ultrasonicbath and finally dried in ambient N2. Before deposition, both theTiO2 and Ag targets were presputtered for 30 min to remove anycontaminants. TiO2 and Ag were deposited using RF powers of 90and 30 W, respectively. During sputtering, the glass substrate wasconstantly rotated at a speed of 21 rpm for Ag and 14 rpm for TiO2.The thickness of the top and bottom TiO2 films was 45 nm, andthat of the Ag layer was fixed at 17 nm [27]. Note that the bottomTiO2 films on glass substrates were annealed in a furnace at dif-ferent temperatures of 300 and 600 °C for 1 h in air to modify theirn values; the Ag and upper TiO2 layers were consecutively de-posited on top of these films. The thicknesses of the multilayerfilms were determined using high-resolution transmission elec-tron microscopy (HR-TEM, JEM-ARM 200 F, JEOL), as demonstratedin Fig. 1. A bare glass substrate was used as a reference whenmeasuring the optical transmittance of the films. The crystalstructure of the annealed TiO2 films was determined using XRD(D8-Advance, BRUKER, Germany). The n values of the sampleswere measured using an ellipsometer (SE MG-1000, NANO-VIEWCo., Ltd., Korea). To measure n, the TiO2 films were deposited on Sisubstrates, followed by annealing at various temperatures. Thetransmittance of the multilayers was measured using a UV/visiblespectrometer (UV-1800, Shimadzu, Japan). Hall measurementsusing the van der Pauw technique were performed under a mag-netic field of 0.55 T (Ecopia HMS 3000, Korea). The four-point-probe technique was used for the sheet resistance measurements.The surface characteristics of the samples were examined usingatomic force microscopy (AFM, XE-100, Park System, Korea).

Fig. 3. Transmittance spectra of TiO2/Ag/TiO2 multilayer films at varioustemperatures.

3. Results and discussion

Fig. 2 presents XRD patterns obtained from 45-nm-thick TiO2

films annealed at various temperatures. Both the as-deposited and300 °C-annealed TiO2 samples appear to be amorphous. For thesample annealed at 600 °C, diffraction peaks appear at 2θ¼25.3°

J.-H. Kim et al. / Ceramics International 42 (2016) 14071–14076 14073

and 48.1°, which correspond to the (101) and (200) planes of TiO2,respectively (JCPDS No. 07–0701); both peaks are indicative of theformation of the anatase TiO2 phase.

Fig. 3 presents the transmittance spectra of the TiO2/Ag/TiO2

(45 nm/17 nm/45 nm) multilayer films for various temperatures.Regardless of the annealing temperature, the transmittancereaches a global maximum at a wavelength of approximately570 nm and then slowly decreases with increasing wavelength.The as-deposited multilayer film exhibits a transmittance of 94.5%at 550 nm, whereas the multilayer films annealed at 300 and600 °C exhibit transmittances of 94.5% and 96.6%, respectively. TheTiO2/Ag/TiO2 film annealed at 600 °C exhibits the widest trans-mission window and highest transmittance in the 400–800 nmregion. The 600 °C-annealed multilayer film exhibits the highesttransmittance of 96.8% at 560 nm. It is noteworthy that the opticalbandgaps of the TiO2/Ag/TiO2 films slightly increase with in-creasing temperature, as observed in the inset of Fig. 3, which isconsistent with the temperature dependence of the transmittance.Although the dependence of the optical bandgap on temperatureis currently not clearly understood, it may be explained as follows.Naik et al. [28] investigated the optical properties of TiO2 thin filmsdeposited at room temperature by RF magnetron sputtering as afunction of the total pressure of the sputtering gases (ArþO2) andthe substrate bias. They showed that the optical bandgap de-pended on the presence of different types of phases. In otherwords, the TiO2 with anatase phase had a higher bandgap thanthose with amorphous and rutile phases. This finding may explainwhy the 600 °C-annealed sample had the highest bandgap. Fur-thermore, as discussed later, the 300 °C-annealed sample ex-hibited a higher carrier concentration than the as-depositedsample. Thus, the increased bandgap of the 300 °C-annealedsample may be explained by a Burstein–Moss shift [29].

Fig. 4 plots the n values and extinction coefficients (k) mea-sured from 45-nm-thick TiO2 and Ag films using an ellipsometer.For the Ag film [Fig. 4(a)], n drastically drops at approximately335 nm and then varies insignificantly with increasing wave-length, whereas k continuously increases with increasing wave-length. For the TiO2 films annealed at various temperatures [Fig. 4(b)], n gradually increases up to 450 nm and then rapidly increasesas the wavelength decreases from 450 to 310 nm. This behavior isknown as the dispersion of n [30]. The 600 °C-annealed TiO2 filmhas a higher n value than the other two TiO2 samples [Fig. 4(b)].The TiO2 film exhibited the highest n of 2.336 at 550 nm whenannealed at 600 °C. The annealing-induced modification of n maybe associated with the annealing-induced grain growth, whichincreases the packing density of the films [31] and the phasetransition from amorphous to anatase TiO2 [2,3]. The estimated n

Fig. 4. n and k values for (a) Ag an

values are similar to those reported by other researchers [32]. Forinstance, Won et al. [32] investigated the effect of rapid thermalannealing on the optical properties of metalorganic chemical-va-por-deposition-grown TiO2 films (550-nm thick) and reported thatn of the as-deposited TiO2 film was approximately 2.0 at 500 nm.The n values of the TiO2 films annealed at 800 °C are 2.2–2.6 in thewavelength range from 500 to 900 nm. All three of the TiO2 filmshave the same cut-off wavelength of k¼0 at λ¼400 nm. It is notedthat in this study, the bottom TiO2 film on the Si substrate wasannealed at various temperatures. However, no interfacial reac-tions between TiO2 and Si are expected to occur because the for-mation enthalpy of anatase TiO2 is lower than that of SiO2 [33,34].

Fig. 5(a) plots the carrier concentration and Hall mobility of theTiO2/Ag/TiO2 multilayer films annealed at various temperatures.The carrier concentrations were measured to be 7.9, 8.2, and8.0�1021 cm�3 for as-deposited, 300 °C-annealed, and 600 °C-annealed samples, respectively. The mobility was 22.371.3,19.971.3, and 22.0671.3 cm2 V–1 s–1 for as-deposited, 300 °C-annealed, and 600 °C-annealed samples, respectively. The similarconcentrations and mobilities are believed to be associated withthe fact that the middle Ag film is the main current path and thebottom TiO2 film was annealed at different temperatures. Fig. 5(b) plots the resistivity and sheet resistance of the TiO2/Ag/TiO2

multilayer films annealed at various temperatures. All of themultilayer samples show similar sheet resistances; it was3.270.15, 3.5770.15, and 3.3170.15Ω/sq. for as-deposited,300 °C-annealed, and 600 °C-annealed samples, respectively. All ofthe multilayer samples also have similar resistivities; it was3.4270.17, 3.8270.17, and 3.5470.17�10–5 Ω-cm for as-de-posited, 300 °C-annealed, and 600 °C-annealed samples,respectively.

Fig. 6 plots the calculated FOMs (ϕTC) of the TiO2/Ag/TiO2

multilayer films as a function of the annealing temperature. ϕTC

was calculated using the equation defined by Haacke [35],ϕTC¼Tav

10/Rs, where Rs is the sheet resistance and Tav is theaverage optical transmittance (450–700 nm). Tav can be estimatedusing the relation, Tav¼

RV(λ)T(λ)d(λ)/

RV(λ)d(λ), where T(λ) is the

transmittance and V(λ) is the photopic luminous efficiency func-tion defining the standard observer for photometry [11,36]. Asobserved in Fig. 6, the TiO2 multilayer films have very high FOMsranging from 156.9 to 193.9�10�3Ω�1; the highest FOM wasobtained for the multilayer film annealed at 600 °C. To the best ofour knowledge, this FOM is the highest reported thus far for TCOs.

The surface characteristics of the as-deposited TiO2-only(45 nm), annealed TiO2-only, and TiO2/Ag/TiO2 multilayer filmswere examined by AFM. All of the samples exhibited smoothsurface morphologies; those of the TiO2-only samples were

d (b) 45-nm-thick TiO2 films.

Fig. 5. (a) Carrier concentration and Hall mobility and (b) resistivity and sheet resistance of TiO2/Ag/TiO2 multilayer films annealed at various temperatures.

Fig. 6. Calculated FOMs (ϕTC) of TiO2/Ag/TiO2 multilayer films as a function ofannealing temperature.

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characterized by root-mean-square (RMS) roughness values of0.37 and 0.69 nm, as observed in Fig. 7(a) and (b), respectively. Inaddition, the 600 °C-annealed TiO2/Ag/TiO2 multilayer sampleexhibited a RMS roughness of 1.09 nm, as shown in Fig. 7(c). As theuniformity of an organic layer is directly dependent on the surfacesmoothness of the underlying electrodes, the smooth surfaces

Fig. 7. AFM images of (a) as-deposited TiO2-only (45 nm), (b)

indicate that the TiO2/Ag/TiO2 multilayer could be used as a TCEfor organic photonic devices.

To further understand the optical transmittance of the TiO2/Ag/TiO2 structures, scattering matrix method simulations [37] wereconducted. The transmittances were calculated using the opticalconstants (n, k) of the TiO2 and Ag films that were measured in thisstudy, as shown in Fig. 4. In the simulations, the thicknesses ofTiO2 and Ag films were fixed at 45 and 17 nm, respectively. Asdescribed previously, the top TiO2 film was not annealed, whereasthe bottom TiO2 films were annealed at 300 and 600 °C. The cal-culated spectra [Fig. 8(a)] are consistent with the measured ones(Fig. 3), revealing high transmittance over 90% at green-to-redwavelengths. Such high transmittance from the O/M/O structurescontaining an optically thick Ag film can be explained as a con-sequence of destructive interference for partially reflected waves[37]. Notably, the overall transmittance was slightly enhancedwhen the bottom TiO2 film was annealed at 600 °C. The trans-mittance (at 550 nm) of the TiO2/Ag/TiO2 films increases with in-creasing n of the bottom TiO2 films, as illustrated in Fig. 8(b). Thecalculation indicates that the transmittance slowly increases andreaches a maximum when n reaches approximately 2.4.

4. Summary

We developed TiO2/Ag/TiO2 multilayer electrodes with highFOM by modifying n of TiO2. n increased with increasing

annealed TiO2-only, and (c) TiO2/Ag/TiO2 multilayer films.

Fig. 8. (a) Calculated spectra of TiO2/Ag/TiO2 multilayer films annealed at various temperatures. (b) Transmittance (at 550 nm) of TiO2/Ag/TiO2 films as a function of n.)

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temperature. The 600 °C-annealed TiO2 multilayer samples ex-hibited higher transmittance at 550 nm than the as-deposited TiO2

multilayer sample. The 600 °C-annealed sample exhibited thehighest transmittance of 96.8% at 560 nm, a sheet resistance of3.31Ω/sq, and smooth surfaces with an RMS roughness of1.09 nm. The Haacke's FOM was highest for the 600 °C-annealedsample. The results indicate that the modification of n could be apotentially important process for fabricating conducting trans-parent multilayer electrodes for display applications.

Acknowledgements

This work was supported by the Korea Evaluation Institute ofIndustrial Technology (KEIT) (Grant no. 10049601) and the WorldClass 300 Project (Grant no. S2317456) of the SMBA (Korea). S.-K.K. was supported by the Basic Science Research Program throughthe National Research Foundation of Korea (NRF), which is fundedby the Ministry of Science, ICT, and Future Planning (NRF-2013R1A1A1059423).

References

[1] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Flexiblelight-emitting diodes made from soluble conducting polymers, Nature 357(1992) 477–479.

[2] Q. Wan, E.N. Dattoli, W. Lu, Transparent metallic Sb-doped SnO2 nanowires,Appl. Phys. Lett. 90 (2007) 222107.

[3] W.S. Liu, W.K. Chen, K.P. Hsueh, Transparent conductive Ga-doped MgxZn1�xOfilms with high optical transmittance prepared by radio frequency magnetronsputtering, J. Alloy Compd. 552 (2013) 255–263.

[4] T. Minami, H. Sonohara, T. Kakumu, S. Takata, Physics of very thin ITO con-ducting films with high transparency prepared by DC magnetron sputtering,Thin Solid Films 270 (1995) 37–42.

[5] V. Bhosle, A. Tiwari, J. Narayan, Metallic conductivity and metal-semi-conductor transition in Ga-doped ZnO, Appl. Phys. Lett. 88 (2006) 032106.

[6] E.V. Johnson, P. Prod’homme, C. Boniface, K. Huet, T. Emeraud, P. Roca i Ca-barrocas, Excimer laser annealing and chemical texturing of ZnO:Al sputteredat room temperature for photovoltaic applications, Sol. Energy Mater. Sol. Cells95 (2011) 2823–2830.

[7] S.X. Zhang, S. Dhar, W. Yu, H. Xu, S.B. Ogale, T. Venkatesan, Growth parameter-property phase diagram for pulsed laser deposited transparent oxide con-ductor anatase Nb:TiO2, Appl. Phys. Lett. 91 (2007) 112113.

[8] Ö.D. Coşkun, S. Demirela, The optical and structural properties of amorphousNb2O5 thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 277(2013) 35–39.

[9] H. Han, N.D. Theodore, T.L. Alford, Improved conductivity and mechanism ofcarrier transport in zinc oxide with embedded silver layer, J. Appl. Phys. 103(2008) 013708.

[10] D.R. Sahu, S.-Y. Lin, J.-L. Huang, ZnO/Ag/ZnO multilayer films for the applica-tion of a very low resistance transparent electrode, Appl. Surf. Sci. 252 (2006)

7509–7514.[11] S. Yu, W. Zhang, L. Li, D. Xu, H. Dong, Y. Jin, Optimization of SnO2/Ag/SnO2 tri-

layer films as transparent composite electrode with high figure of merit, ThinSolid Films 552 (2014) 150–154.

[12] A. Dhar, T.L. Alford, Optimization of Nb2O5/Ag/Nb2O5 multilayers as trans-parent composite electrode on flexible substrate with high figure of merit, J.Appl. Phys. 112 (2012) 103113.

[13] H.-H. Kim, E.-M. Kim, K.-J. Lee, J.-Y. Park, Y.-R. Lee, D.-C. Shin, T.-J. Hwang, G.-S. Heo, TiInZnO/Ag/TiInZnO multilayer films for transparent conducting elec-trodes of dye-sensitized solar cells, Jpn. J. Appl. Phys. 53 (2014) 032301.

[14] M. Makha, L. Cattin, Y. Lare, L. Barkat, M. Morsli, M. Addou, A. Khelil, J.C. Bernède, MoO3/Ag/MoO3 anode in organic photovoltaic cells: influence ofthe presence of a CuI buffer layer between the anode and the electron donor,Appl. Phys. Lett. 101 (2012) 233307.

[15] K. Jeon, H. Youn, S. Kim, S. Shin, M. Yang, Fabrication and characterization ofWO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 forpolymer light-emitting diodes, Nanoscale Res. Lett. 253 (2012) 1–7.

[16] N. Zhang, Y. Hu, X. Liu, Transparent organic thin film transistors with WO3/Ag/WO3 source-drain electrodes fabricated by thermal evaporation, Appl. Phys.Lett. 103 (2013) 033301.

[17] D. Miao, S. Jiang, S. Shang, Z. Chen, Highly transparent and infrared reflectiveAZO/Ag/AZO multilayer film prepared on PET substrate by RF magnetronsputtering, Vacuum 106 (2014) 1–4.

[18] J.-A. Jeong, H.-K. Kim, Al2O3/Ag/Al2O3 multilayer thin film passivation pre-pared by plasma damage-free linear facing target sputtering for organic lightemitting diodes, Thin Solid Films 547 (2013) 63–67.

[19] J.-H. Song, J.-W. Jeon, Y.-H. Kim, J.-H. Oh, T.-Y. Seong, Optical, electrical, andstructural properties of ZrON/Ag/ZrON multilayer transparent conductor fororganic photovoltaics application, Superlattice Microstruct. 62 (2013) 119–123.

[20] N.M. Le, B.-T. Lee, ZnMgBeO/Ag/ZnMgBeO transparent multilayer films withUV energy bandgap and very low resistance, Ceram. Int. 42 (2015) 5258–5262.

[21] J.C.C. Fan, F.J. Bachner, Transparent heat mirrors for solar-energy applications,Appl, Optics 15 (1976) 1012–1017.

[22] J. Kulczyk-Malecka, P.J. Kelly, G. West, G.C.B. Clarke, J.A. Ridealgh, K.P. Almtoft,A.L. Greer, Z.H. Barber, Investigation of silver diffusion in TiO2/Ag/TiO2 coat-ings, Acta Mater. 66 (2014) 396–404.

[23] A. Dhar, T.L. Alford, High quality transparent TiO2/Ag/TiO2 composite electrodefilms deposited on flexible substrate at room temperature by sputtering, APLMat. 1 (2013) 012102.

[24] I. Dima, B. Popescu, F. Iova, G. Popescu, Influence of the silver layer on theoptical properties of the TiO2/Ag/TiO2 multilayer, Thin Solid Films 200 (1991)11–18.

[25] J.H. Jia, P. Zhou, H. Xie, H.Y. You, J. Li, L.Y. Chen, Study of optical and electricalproperties of TiO2/Ag/TiO2 multilayers, J. Korean Phys. Soc. 44 (2004) 717–721.

[26] K. Hashimoto, H. Irie, A. Fujhishima, TiO2 photocatalysis: A historical overviewand future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269.

[27] J.H. Kim, D.-H. Kim, T.-Y. Seong, Realization of highly transparent and lowresistance TiO2/Ag/TiO2 conducting electrode for optoelectronic devices, Cer-am. Int. 41 (2015) 3064–3068.

[28] V. M. Naik, D. Haddad, R. Naik, J. Benci, G. W. Auner, Optical properties ofanatase, Rutile and Amorphous Phases of TiO2 Thin films grown at roomtemperature by rf magnetron sputtering, in: Proceedings of Materials Re-search Society Symposium, 755, DD11.12.1, 2003.

[29] E. Burstein, Anomalous Optical Absorption Limit in InSb, Phys. Rev. 93 (1954)632.

[30] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed.,Wiley, New York, 1975.

[31] B.D. Fabes, D.P. Birnie, B.J.J. Zelinski, Porosity and composition effects in sol-gelderived interference filters, Thin Solid Films 254 (1995) 175–180.

J.-H. Kim et al. / Ceramics International 42 (2016) 14071–1407614076

[32] D.-J. Won, C.-H. Wang, H.-K. Jang, D.-J. Choi, Effects of thermally inducedanatase-to-rutile phase transition in MOCVD-grown TiO2 films on structuraland optical properties, Appl. Phys. A 73 (2001) 595–600.

[33] M.W. Chase Jr., J. Phys. Chem. Ref. Data Monogr. 9 (1998) 1–1951.[34] M.W. Chase Jr., C.A. Davies, J.R. Downey Jr, D.J. Frurip, R.A. McDonald, A.

N. Syverud, J. Phys. Chem. Ref. Data 14 (Suppl. 1) (1985) S1.

[35] G. Haacke, New figure of merit for transparent conductors, J. Appl. Phys. 47(1976) 4086.

[36] W.G. Driscoll, W. Vaughan, Handbook of Optics, McGraw-Hill, New York, 1978.[37] H.-K. Lee, J.-Y. Na, Y.-J. Moon, T.-Y. Seong, S.-K. Kim, Design of near-unity

transmittance dielectric/Ag/ITO electrodes for GaN-based light-emittingdiodes, Curr. Appl. Phys. 15 (2015) 833.